The objective of the proposed research is to investigate certain aspects of fertilization in mammals. Firstly, we propose to study the process of sperm penetration through the zona pellucida, the relatively thick acellular membrane enclosing the egg. How spermatozoa penetrate this final barrier to reach the vitellus is still largely unknown. Shortly after sperm penetration and sperm-egg fusion, the oocyte completes its final meiotic division, resulting in the formation of the second polar body. This process will also be studied in detail: almost nothing is known about this unusual, unequal form of cell division. Sperm penetration and second polar body formation will be examined in two species, the rat and guinea pig, by the continuous observation and filming of living, freshly collected eggs in vitro. Data will be obtained on the timing of these and related events in fertilization. Time-lapse and high-speed cinematography, combined with phase and polarizing microscopy, will be employed to obtain a visual and quantitative record of the events and to investigate their essential nature. This research will provide basic information on some of the fundamental steps in fertilization. Such information could prove to be not only pertinent but actually essential for future work on the fertilization of human eggs in vitro.